Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. For example, in the imaging field, CMOS image sensors have proved to be less expensive to manufacture relative to CCD imaging devices. Further, for certain applications CMOS devices are superior in performance. The pixel elements in a MOS device can be made smaller and therefore provide a higher resolution than CCD image sensors. In addition, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand alone imaging device.
Examples of MOS imaging devices are detailed in “A ¼ Inch Format 250K Pixel Amplified MOS Image Sensor Using CMOS Process” by Kawashima et al., IEDM 93-575 (1993), and “A Low Noise Line-Amplified MOS Imaging Devices” by Ozaki et al., IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991. In addition, U.S. Pat. No. 5,345,266 to Denyer titled “Matrix Array Image Sensor Chip” describes a MOS image sensor. CMOS image sensors are now available from a variety of manufacturers, including the assignee of the present invention, OmniVision Technologies, Inc.
The primary building block of an image formed by a CMOS image sensor is a pixel. The number, size and spacing of the pixels determine the resolution of the image generated by the imaging device. The pixels of a CMOS image sensor are semiconductor devices that transform incident light photons into current signals. The signal produced by each pixel is generally extremely small.
One important parameter that the CMOS image sensor must be able to control is the exposure time of each pixel to incident light. Similar to light exposure time for photographic film, the exposure time of each pixel must be adjusted to compensate for variations in lighting conditions, such as for indoor or outdoor lighting. An exposure time that is too long will result in an image that is overly bright and washed out. In contrast, an exposure time that is too short will result in an image that is dark and difficult to view.
Another method of varying the intensity of the pixel signals is to adjust the gain of the amplification circuitry. In many ways, controlling the gain is substantially similar to controlling the exposure time. By varying one or both, the image generated by the image sensor can be optimized for viewing.
U.S. Pat. No. 5,734,426 describes one prior art method of controlling exposure time. While adequate for many applications, the speed at which the exposure time can be automatically changed is insufficient for certain applications. For example, for automobile applications, where the automobile may travel from sunlight, to darkness (such as in a tunnel), back to sunlight, it is important for the image sensor to be able to adjust its exposure time quickly.